The invention relates to the field of the insulation of rotating electrical machines. In particular, the invention relates to a process for producing a high-quality insulation for conductors or conductor bundles as are used in rotating machines, for example in the form of stator coils, transposed bars and excitation conductors.
Various processes are customarily used in the field of insulation of conductors or conductor bundles of rotating electrical machines.
In one process, tapes comprising a glass-fiber support and mica paper are wound helically in layers onto a stator conductor until a desired insulation thickness is reached. Subsequent impregnation in epoxy resin displaces residual air from the insulating winding formed in this way, and the layers of tape are adhesively bonded. Curing in a suitable mold imparts the final shape to the insulation. For production reasons, in this process the mica platelets are oriented in the direction of the tape, which in the finished insulation results in the mica platelets being oriented parallel to the conductor surface.
According to a further process, which is known from EP 0 660 336 A2, tapes consisting of thermoplastic filled with mica are wound around stator conductors. Consolidation and shaping in this case take place by means of hot pressing of the stator conductor around which the tape has been wound, during which process air is displaced, the thermoplastic is melted and the layers of the winding are adhesively bonded. In this process too, the mica platelets are oriented parallel to the conductor surface.
Finally, the stator conductor can also be insulated by extrusion of thermoplastics without fillers, i.e. also without mica, as described in U.S. Pat. No. 5,650,031.
Nowadays, however, the conductors of rotating electrical machines which are to be insulated are generally structures of a very complex shape, in the form of bars or coils. A straight part of the conductors is located in the grooves of the stator of the machine. A curved part of the conductors, after suitable connection to adjacent bars and coils, forms a winding head which projects out of the stator at both ends. In the case of large rotating machines, the length of the straight part may exceed 6 m. A problem hitherto has been that insulation and conductor usually have different coefficients of thermal expansion xcex1 which, over the course of time, on account of thermal stresses, may lead to defects in the insulation as a result of cavities which form where the insulation becomes detached, and that defects, for example inclusions of air, are formed during the production of the insulation. Partial discharges may occur at such defects, leading to damage to the insulation. In this case too, partial discharge activities in the 100 nC range are quite customary.
In view of these partial discharge activities, hitherto it has only been possible for the machine insulation to operate reliably as a result of the barrier action of mica platelets oriented perpendicular to the field direction. This prevents the formation of flashover passages leading out of the cavities. 2.5 to 2.75 kV/mm is generally regarded as the upper limit for long-term reliability of the operating field strength. However, a maximum level such as this is exceeded, in some cases considerably, by other insulation systems used in medium- or high-voltage insulation.
For example, the maximum field for long-term operation in pin-type insulators, in which an alumina-filled epoxy resin is used for gas-insulated circuits, is 4 kV/mm, and the maximum field for high-voltage cables, in which polyethylene is used, is approx. 12 kV/mm. A common feature of these conventional insulation systems is that there are no partial discharges under operating load. However, application of these insulation systems for machine insulation is not possible for the application according to the invention, since in those cases the insulation is only subject to weak electrical loads and therefore freedom from defects in accordance with the invention is not of crucial importance, and consequently defects are acceptable in this case. Other conventional insulation techniques are not suitable for the demands imposed by the application according to the invention.
However, since, moreover, the conventional processes and materials using mica which are currently in use are substantially already more than thirty years old, at best incremental improvements are to be expected from any further developments to this prior art. Therefore, it appears highly unlikely that it will be possible to further develop this prior art to develop a higher-quality insulation which can be produced with shorter throughput times and lower manufacturing costs compared to the prior art, and also in an environmentally friendly production process, i.e. without the use of solvents, without emissions and without the production of special waste, and which does not include any defects or, if there are defects, these defects do not lead to any partial discharges.
Therefore, it is an object of the present invention to provide a process for producing a high-quality insulation for conductors or conductor bundles which is of a high quality and can be produced with short throughput times and low manufacturing costs and in an environmentally friendly fashion.
According to the invention this object can be achieved by processes for producing a high-quality insulation for conductors or conductor bundles as described herein.
This inventive process for producing a high-quality insulation for conductors or conductor bundles without defects which may lead to partial discharges under test and operating loads means that the oriented mica platelets are no longer required. This greatly facilitates both the choice of production processes and the choice of materials for the insulation, since for many polymers it is difficult to incorporate mica in concentrations of more than 40 percent by weight.